Where each of a collection of computers or other instruments is to be controlled or driven by a sequence of timing signals, each such instrument is often left to provide its own timing signals by independent timing means, usually by means of an internal clock. These timing means are often not initially synchronized with each other, and the timing signals produced by any two of these timing means can drift relative to each other with the passage of time.
Some workers have attempted to distribute timing signals for two or more computers or peripherals, using an internal clock or an external clock, which is subject to drift.
U.S. Pat. No. 3,520,128, issued to Novikov et al, discloses an automatic time distribution system. An independent primary clock is connected to, and provides exact time signals for, a plurality of secondary clocks by radio waves. Each secondary clock receives a sequence of uncorrected "exact" time signals and a sequence of timing marks to correct this uncorrected time. The time signals for each secondary clock are apparently corrected separately.
Entner discloses an aircraft navigation system that employs time synchronization provided by satellite-ground communications, in U.S. Pat. No. 3,643,259. A ground-based observation station monitors and predicts the future positions of a single satellite orbiting the Earth and transmits this information to the satellite, which receives and stores this information. The satellite and an aircraft, for which navigation information is to be supplied, have precise on-board clocks, synchronized with each other. The aircraft transmits a first pulse, which is received by the satellite after a time interval .DELTA.t1, and causes the satellite to transmit a second pulse, which is received by the aircraft after a further time interval .DELTA.t2. The total time difference .DELTA.t1+.DELTA.t2 between transmission of the first pulse and receipt of the second pulse at the aircraft determines the distance travelled by the first pulse. The aircraft generates a first sphere, whose radius is the distance travelled by the satellite signal to the aircraft, and a second sphere, whose center is displaced from the first sphere center by the distance travelled by the aircraft in the time interval of length .DELTA.t1. A third sphere is generated in a manner similar to generation of the first sphere. The (point) intersection of the three spherical surfaces determines the position of the satellite, and the location of the aircraft relative to the satellite is then determined.
Cater, in U.S. Pat. No. 3,811,265, discloses transmission of coded, time-indicating signals from a master clock at a central station to one or more slave clocks, using a two-wire line and binary-valued pulses with different time durations. A time synchronizing pulse is periodically inserted (e.g., once per second) on the line to correct for drift or other errors. If the two-wire line is a standard 60-cycle power line or a television cable, the binary-valued pulses use one or more frequencies that lie outside the frequency range normally used on that line, to avoid signal interference with the standard signals transmitted over that line.
A clock that can be synchronized by "wireless" signals is disclosed by Gerum et al in U.S. Pat. No. 3,881,310. The clock contains an electromagnetically operated mechanical oscillator whose frequency 2f0 is twice the rated frequency of an alternating current network connected to the clock. A time synchronization module transmits a signal of frequency f1&gt;&gt;f0 that is modulated by the network at a frequency f=2f0 and is received and demodulated by the clock. Normally, the pulses received from the network drive the clock, and the oscillator is in a standby mode. The clock oscillator is enabled, and the network is disconnected, when and only when the network frequency differs by at least a predetermined amount from the frequency 2f0 of the oscillator. The oscillator in standby mode receives resonance energy of frequency.apprxeq.2f0 from the network for maintaining the oscillations.
A TACAN air navigation system is disclosed in U.S. Pat. No. 3,969,616, issued to Mimken. Range of an aircraft from an interrogation signal-transmitting beacon is determined by the lapse in time between transmission of the interrogation signal and receipt of a reply pulse signal from the aircraft (called a "dwell" period in TACAN parlance). A circuit at the beacon generates and uses a filler pulse during any dwell period in which a reply pulse is not received from a target aircraft, in order to maintain a rough and unspecified synchronization at the beacon for the target aircraft when reply pulses are not received. An aircraft velocity detector may be included, with velocity being determined by averaging over several successive dwell periods to reduce the associated velocity error.
Cateora et al, in U.S. Pat. No. 4,014,166, disclose a satellite-controlled digital clock system for maintaining time synchronization. A coded message containing the present time and satellite position is transmitted from a ground station to an orbiting satellite and is relayed to a group of ground-based receivers. A local oscillator aboard the satellite is phase-locked to a precise frequency to provide the system with accurate time-of-year information by a count of the accumulated pulses produced by the oscillator. This count is compared with a time count determined from the coded message received by the satellite. After a selected number of errors are observed through such comparisons, the on-board clock is reset to the time indicated by the coded messages received. If transmission of the coded messages is interrupted, the on-board oscillator continues to provide time information that is transmitted to the ground-based receivers.
An antenna space diversity system for TDMA communication with a satellite is disclosed by U.S. Pat. No. 4,218,654, issued to Ogawa et al. Differences of temporal lengths of paths from the satellite through each antenna to a ground-based signal processor station are determined by measurement of times required for receipt of pre-transmission bursts sent in the respective allocated time slots through two different antennas, in a round trip from base station to satellite to base station. Variable time delays are then inserted in the base station signal processing circuits to compensate for the temporal length differences for the different signal paths. These time delays are changed as the satellite position changes relative to each of the antennas.
U.S. Pat. No. 4,287,597, issued to Paynter et al, discloses receipt of coded time and date signal from two geosynchronous satellites, which signals are then converted into local date and time and displayed. The frequency spectrum is scanned by an antenna to identify and receive the satellite signals. Temporal length differences for signal paths from each satellite through a receiving antenna to a signal processing base station are determined, to provide compensation at the base station for these differences. Time information is provided by a satellite every 0.5 seconds, and this information is corrected every 30 seconds. Signals from either or both satellites are used to provide the time and date information, in normal local time and/or daylight savings local time.
Jueneman discloses an open loop TDMA communications system for spacecraft in U.S. Pat. No. 4,292,683. A spacecraft, such as a satellite, in quasi-geosynchronous orbit carries a transponder that relays a coded signal from a ground-based signal-transmitting station to a plurality of spaced apart, ground-based receivers. This coded signal includes a time index and an index indicating the spacecraft's present position. The time index is adjusted by each receiver to compensate for the changing position of the spacecraft through which the coded signal is relayed. The system is open loop and requires no feedback from the receivers to the base station.
Method and apparatus for determining the elapsed time between an initiating event and some other event are disclosed by U.S. Pat. No. 4,449,830, issued to Bulgier. A first timer and a second time mark the times of occurrence, respectively, of an initiating event and a subsequent event that depends upon occurrence of the initiating event. The two timers are initially connected and synchronized, then disconnected before the initiating event occurs. The timers are then reconnected after both events have occurred, to allow determination of the elapsed time between occurrence of the two events.
Distance ranging and time synchronization between a pair of satellites is disclosed by Schwartz in U.S. Pat. No. 4,494,211. Each satellite transmits a timing signal and receives a timing signal from the other satellite. The difference in time, including compensation for signal processing delay on a satellite, between transmission and receipt of the signals is transmitted by each satellite to the other satellite and is used to establish time synchronization and to determine the distance between the two satellites. This exchange of signals would be repeated at selected time intervals to maintain synchronization, where the satellites are moving relative to each other. No communications link to a third entity is required, and only one of the satellite clocks is adjusted to establish and maintain time synchronization.
Plangger et al, in U.S. Pat. No. 4,582,434, disclose transmission and receipt of a continuously corrected sequence of timing signals. A microprocessor at the receiver periodically compares these timing signals with on-board timing signals generated by a local clock. A varactor diode in a crystal oscillator circuit adjusts the microprocessor's operating frequency to minimize, but not necessarily eliminate, any error between the two timing signal sequences. Delay time for timing signal processing is compensated for in a receiver circuit. The frequency for microprocessor operation is continuously corrected. If the transmitted timing signals are too weak, or do not arrive, the on-board timing signals are used to control the microprocessor until the transmitted timing signals are received in sufficient strength again. This approach uses Greenwich Mean Time, updated approximately once per minute to reduce the timing inaccuracy to a maximum of 0.1 sec.
Noguchi discloses a remote time calibration system using a satellite, in U.S. Pat. No. 4,607,257. A base station provides a reference system of absolute timing signals and transmits these to a satellite that orbits the Earth. The satellite then calibrates and periodically adjusts its internally generated time and transmits observed data plus the corresponding adjusted satellite time to one or more data receiving stations on the Earth that are distinct from the base station. Time calibration optionally compensates for signal propagation time delay from base station to satellite and allows continuous transmission of data from satellite to the data receiving station(s). Several time difference indicia are computed here.
A local area network, having a master timing subsystem and having a plurality of modules, each with a separate timing subsystem, is disclosed by Kirk in U.S. Pat. No. 4,890,222. Each timing subsystem has three timing sequences(fine resolution, synchronization, and present time to the nearest second), each having a different period. Each module timing subsystem can independently provide timing signals for its associated module, or timing signals for all modules can be synchronized by the master timing subsystem. A master timing frame is periodically transmitted by the master timing subsystem to each of the modules; this master timing frame is received and used by each module as its sole timing signal source only if certain conditions are present.
Lusignan, in U.S. Pat. No. 4,972,507, discloses a cellular communications protocol and system in which a plurality of user station respond to transmission command from a base station. The response intervals for each user station are staggered and synchronized so that no overlap occurs between transmissions from different user stations. A unique time delay for each user station is assigned and broadcast by the base station at selected times. However, the base station does not continually provide a sequence of timing signals for synchronization of the user station times inter se.
A time slot allocation method for in a TDMA or similar network is disclosed in U.S. Pat. No. 5,062,035, issued to Tanimoto et al. Time is synchronized, and time slots are allocated and re-allocated, by a master station that communicates directly with each subsidiary station. If the number of reserve or unused time slots of a subsidiary station (1) falls below a first threshold or (2) rises above a second threshold, the master station (1) supplements the time slots of that subsidiary station with one or more of its own time slots or (2) re-allocates to itself one or more time slots from that subsidiary station, respectively. This approach reduces the wait time for signal transmission on the network.
U.S. Pat. No. 5,072,442, issued to Todd, discloses a teleconferencing network in which the stations, operating at different clock rates, can communicate with each other synchronously. A TDMA bus connecting the stations operates at a clock rate equal to the number of stations times the highest clock rate for all the stations, divided by the bit width of the TDMA bus. Buffers are used to receive and hold signals for transmission and/or processing. This approach appears to require redetermination of the bus rate each time the number of network stations changes.
A phase adjustment system for communication between a central station and a plurality of transmitter-receiver pairs is disclosed by Nakahara in U.S. Pat. No. 5,077,759. A target transmitter identification number is transmitted as part of a turn-on signal by the central station, received by the associated receiver, received by the target transmitter from that receiver after a selected time delay, and transmitted to the central station. The central station then adjusts the time delay of each transmitter-receiver pair to achieve a chosen phase relationship of the transmitter-receiver pairs relative to the central station.
Durboraw, in U.S. Pat. No. 5,119,504, discloses use of GPS to assign a mobile, ground-based receiver of GPS signals to a "cell", defined by one or more GPS satellites, on the Earth's surface for communications purposes. The ephemerides for the satellites are stored in the receiver, or in a ground-based system with which the receiver communicates. This patent appears to contemplate two-way communication between a satellite and the receiver provide receiver timing information and to implement receiver hand-offs between cells.
These approaches do not provide a time distribution that is substantially the same for all points on the Earth's surface and do not continually provide corrections to the timing signals issued to and used by the recipient instruments. Further, these approaches are often quite specialized and are therefore difficult to extend to an arbitrary number of instruments or other users. What is needed is a time distribution system that: (1) is usable anywhere on or adjacent to the Earth's surface; (2) is easily extended to any number of time-controlled instruments or other users; (3) continually provides corrections in the timing signals distributed; (4) can provide periodic timing signals and/or special event timing signals on demand; and (5) operates independently of the number of users.